1. Field of the Invention
The present invention relates to an apparatus for recording and reproducing information on tape. The invention is particularly related to a technique for controlled, bidirectional movement of tape, without switching, and requiring only one power source to drive the motive elements.
2. Description of the Prior Art
Tape drives for retrievable, informational data are common features of computer and other data processing systems.
It is a feature of their usage that they be able to cause the tape to move forwards or backwards, past the head in a controlled fashion, stopping and starting at intervals, as data is recorded or retrieved, bidirectionally, from among a pleurality of data blocks which are laid down along the length of the tape. The stopping and starting of the tape must be accomplished in a set, short distance, this being the length of tape between data blocks, itself devoid of data records, so that data may only pass the head when the tape is at a correct operational speed.
Because of tape wear problems, and inertia problems, the older art of capstan driven tape transports has generally been abandoned in favour of `reel-to-reel` techniques. In these techniques, there is no capstan present, the tape being caused to move past the head in a controlled fashion by means of torque, applied directly to the two tape storage reels by two motors one driving each reel, the two motors themselves under the control of a tape speed control servo mechanism, responsive to signals from a tape file control unit.
The design of the tape speed control servomechanism, has, in the past, been beset by problems concerning the implementation of bidirectional, controller drive. Because the tape must be able to stop and start in the same short distance in either direction, the response time of the servo must be substantially the same no matter which of the two reels is taking up or paying out the tape. In order to accomplish this end, it has been usual to mount a rotational speed sensing tachometer onto each reel turning motor, so that either tachometer may be used as the speed sensing element of the tape speed control servo. Much switching of common elements, or wastage, through only partial use, of unique elements, between the two motors, has been necessary to `turn the servo round` in this manner.
In addition, because the diameter of the reels varies as the tape moves from one reel to the other, a fixed magnitude speed demand signal, from the tape drive control unit to the tape speed control servo, produces a different tape speed for one direction of tape travel compared to the tape speed produced for the other direction of tape travel, since the actual quantity controlled in the angular velocity of the monitored motor, and the linear tape speed is a linearly increasing function of the radius of the reel on that motor. The two reels being of different radii, the tape speeds for opposing directions will be different. Recomputation of speed demand signal by the tape drive controller then becomes necessary. A further disadvantage of the prior art has been the necessity for dual polarity power sources for the reel driving motor. This has complicated the amplifier design in tape speed control servos and limited the overall economy and utility of tape drives by requiring the availability of more than one high capacity power source.